This invention relates to internal combustion engine ignition analyzers and in particular, to a new and improved method and apparatus for generating sweep signals for the oscilloscope of an analyzer.
Ignition analyzers are utilized at the present time by engine mechanics to provide information regarding the performance of an internal combustion engine, typically the engine for an automobile or truck. A cathode ray oscilloscope is provided for displaying certain types of information, usually with the horizontal sweeps being initiated by an ignition timing event such as points open or points closed or No. 1 plug firing or the like. The usual constant speed sweep is not satisfactory for engine analyzers where the engine speed will vary over a wide range between idling and full speed. It is desirable to have the horizontal sweep utilize the full width of the tube face regardless of sweep repetition rate, which is proportional to engine speed.
Prior art ignition analyzers have attempted to generate a horizontal sweep proportional to the engine speed and typically utilize amplitude comparison circuits. Various ways have been utilized in the past and each has had some limitation. The limitations on the prior art sweep generators usually show up as variation in the sweep length at different engine speeds. It is desirable to have the sweep generating system stable and therefore relatively large time constants are utilized; however, the large time constants tend to limit the ability of the system to synchronize itself with rapid changes in engine speed. Attempts have been made to speed up the synchronization by making the circuitry under damped; this results in a horizontal trace which at first is too long and then too short before settling down to the proper trace length.
It is an object of the present invention to provide a new and improved sweep generator for the oscilloscope of an engine ignition analyzer which provides a horizontal sweep of substantially constant length for a wide range of engine speeds. A particular object is to provide a phase lock loop for generating the ramp voltage for the sweep signal, making a phase comparison between the engine timing signal and the end of the sweep signal, with the slope of the ramp voltage being changed to reduce any difference in phase of the two signals so that the voltage at the end of the ramp (the end of the sweep) remains substantially constant regardless of the frequency of the input signals (the sweep repetition rate).